1. Field of the Invention
This invention pertains in general to fluidized bed reactors and more particularly to fluidized bed reactors that can operate continuously even with a heavy residue yield.
2. Background Information
In the production of metals such as zirconium, hafnium and titanium by the Kroll reduction process, zircon and rutile sands are first chlorinated in the presence of carbon to produce gaseous zirconium, hafnium, titanium and silicon tetrachlorides, which metals are then reduced with, e.g., magnesium. The sands are typically chlorinated in the presence of carbon at temperatures greater than approximately 800xc2x0 C. in fluidized bed reactors. Non-volatile compounds, and particularly calcium, yttrium, and sodium salts in the zircon and rutile sands, eventually build up in the chlorinators and create xe2x80x9cstickyxe2x80x9d bed conditions which inhibit fluidization. Thus, the residues in the chlorinators must be periodically removed. Existing production chlorinators operate in a semi-continuous mode where the temperature, pressure and material feed rate are maintained constant while the residual impurities from the feeds are left to accumulate in the fluidized bed reactors. The residue contains impurities, such as CaCl2 and YCl3, which are in molten states at reactor operating conditions and, if left to accumulate in the bed, will freeze up the bed and eventually require reactor shutdown. During production, this occurs every two to four weeks. Each time about 3,000 pounds of residue, typically containing more than 95% coke, is pulled from the bed. This material is required to be disposed of as radioactive waste because of uranium and thorium which also occur in the ore. This operating mode not only hampers the productivity but also increases the waste of raw material and the amount of residue required to be disposed as radioactive material. In order to permit these chlorinators to operate in a truly continuous mode and avoid bed operational problems, the residue needs to be removed from the bed continuously. This is particularly difficult in current fluidized bed designs that employ an array of rocks, below the bed to diffuse and evenly distribute the gas that suspends and entrains the reactants. In chlorinators silica rocks are chosen as the diffuser because of the corrosive nature of the chlorine gas. On the other hand, the bed of rocks forms a trap for the collection of the sticky residue which readily clogs the flow channels that are intended to distribute and diffuse the chlorine gas.
Accordingly, a new fluidized bed reactor design is desired with an improved gas distribution scheme that can suspend, entrain, and mix the reactants without forming an impediment to the collection and removal of process residue. Furthermore, such a design is desired that will facilitate the continual removal of the process residue.
These and other objects are achieved by this invention, employing an open and unrestricted fluidized bed within the reactor housing without any impediments to the collection and removal of the process residue. A central gas inlet is positioned proximate the bottom of the reaction zone of the fluidized bed reactor housing, for directing gas upwardly through the housing to maintain the raw materials in suspension. A plurality of peripheral gas jets are positioned at at least two elevations along the reactor housing, circumferentially around the housing at each of those elevations, that introduce gas at a defined angle to promote mixing of the entrained materials in suspension. In this way, the reactants are mixed in suspension without positioning a diffuser below the reaction zone. The bottom of the housing is funneled to a collection port to guide the residue under the force of gravity to an exit where the residue can be continuously extracted.
In a preferred embodiment, a gas sparger surrounds an upper portion of the collection zone and introduces the fluidizing gas to direct agglomerates below a predetermined weight back into the reaction zone to increase the efficiency of the process and minimize the residue. In this preferred embodiment, approximately 30% of the fluidizing gas is directed through the central gas stream, 5% through the sparger and 65% through the peripheral gas jets. Individual control of the peripheral gas jets provides operational flexibility.